1. Field of the Invention
This invention relates to a device for supporting an objective lens of an optical pickup in which the objective lens is supported on a base through leaf springs so that the lens can be displaced in both tracking and focusing directions.
2. Brief Description of the Related Art
Generally, in optical pickups, a beam from a light source is projected as a spot onto a rotating optical disc through an objective lens so that information can be recorded onto, and/or reproduced from, the optical disc. Therefore, it is necessary that the objective lens form the light beam into a spot having a constant diameter, and that the light beam spot be projected on an information recording track of the optical disc.
However, due to various factors, there are occasions when the spot deviates from the track, resulting in tracking error, and other occasions when the spot diameter varies, resulting in focusing errors. For this reason, one current procedure for overcoming these problems is to detect such tracking and focusing errors and to drive the objective lens so that it is displaced in the tracking and focusing directions in accordance with the amounts of such errors so as to effect the necessary adjustment. In this manner, the errors in the tracking and focusing directions are corrected to achieve good recording and reproduction.
In one conventional device, in order for the objective lens to be displaced in the tracking and focusing directions, a first leaf spring is secured at one end to a spring mounting portion of a base, and the other end of the first leaf spring is secured to a first spring mounting portion of a connective member. A second leaf spring is secured at one end to a second spring mounting portion of the connective member. In addition, a spring mounting portion of a holder member for holding the objective lens is secured to the other end of the second leaf spring. As a result, the objective lens can be displaced in the tracking direction by one of the two leaf springs and can be displaced in the focusing direction by the other leaf spring.
An exciting current corresponding to the amount of each error, is sent through a coil mounted on the lens holder member, thereby changing the magnetic force produced in this coil. As a result, the objective lens is driven so that it is displaced in the tracking and focusing directions by a combination of the thus changed magnetic force and the magnetic forces of permanent magnets mounted on the base. Similar devices are disclosed in Japanese Laid-Open (Kokai) Patent Application Nos. 5480/87 and 112239/87.
In such a construction, however, since a rigid body mode inevitably remains and the leaf springs supporting the objective lens have respective natural modes, when the oscillation frequency of the objective lens in the tracking or focusing directions coincides with the natural frequency of the rigid body mode or the natural mode of each leaf spring, there occurs a resonance phenomenon which may then adversely affect the objective lens driving characteristics.
One procedure to prevent such a phenomenon, as disclosed in Japanese Laid-Open (Kokai) Patent Application No. 125544/87, is to use a leaf spring having a laminate construction. Such a leaf spring includes a pair of metal leaf spring elements and an adhesive elastic sheet connecting the pair of leaf spring elements together. With this arrangement, the elastic force of the adhesive elastic sheet can restrain the resonance level to a certain degree.
However, since the leaf spring of the laminate construction uses metal leaf spring elements, the natural oscillation mode of the metal leaf spring elements remains, and depending on the residual level of these metal leaf spring elements, this natural oscillation mode adversely affects the objective lens driving characteristics. In addition, the rigid body mode should not be weakened too much as a result of this laminate construction. Further, since the leaf spring of the laminate construction is constructed from sheet-like members having different elastic forces, the leaf spring is liable to be affected by variations in the fundamental objective lens driving characteristics and by environmental variations (for example, a temperature change, a moisture change, etc.). In addition, considerable time and labor are required to manufacture such a leaf spring, and therefore the cost is high.